Fixed vessels (e.g. sinks, baths etc.) for holding liquid generally have an outlet e.g. positioned at the lowest point thereof to allow used liquid to drain away e.g. to a main sewer. In order for the vessel to hold liquid without draining occurring it is necessary to seal the outlet. Sealing can be achieved by stopping the outlet using a rubber disc. The disc can be attached to the vessel by a chain to facilitate removal. Alternatively, the disc can be part of an in-built mechanism operable by a lever, commonly known as a pop-up waste. By sealing the outlet, the risk of overflow of the vessel is created. To avoid this problem, extra outlets are provided in vessels at a certain height from the lowest point. This means that liquid will flow out of these outlets when the height in the vessel is above a predetermined amount. These so-called “overflows” remove the risk of flooding caused by overfilling the vessel.
However, overflow channels have disadvantages. Firstly, they are difficult to clean because by their nature they must include a relatively long enclosed channel. They easily become dirty and can be unsightly. Secondly, they are relatively difficult to manufacture. Traditional bathroom/kitchen vessels are made from china, and the formation of a self-contained channel presents certain design and manufacturing limitations.
Furthermore, more modern vessel designers use different materials, some of which are transparent (e.g. glass). Overflow channels are not provided in these vessels because of the problem of cleaning and unsightliness mentioned above. There is therefore a risk of overfilling these vessels.